This invention relates to a method of pulse-mode driving of a semiconductor laser in which a pedestal current which is smaller than the laser-threshold current and an information current pulse are passed through the laser for obtaining a laser pulse, the sum of the pedesal current and the information current being larger than the laser-threshold current. The invention also relates to a driving device for a semiconductor laser and to an optical writing apparatus provided with a semiconductor laser and such a driving device. Laser-threshold current is understood to mean the minimum current through the laser at which the radiation is predominantly emitted as laser radiation. Within the scope of the present invention information current is understood to mean an electrical current which is passed through the laser in order to generate a laser pulse which corresponds to a unit of information to be recorded.
The said method and devices can be used for recording information in optical record carriers. For example, data originating from an electronic computer or to be applied to such a computer can be stored on these record carriers. Also audio or video programmes may be recorded by a user himself. Examples of such record carriers are known under the names "Digital Recording" or "DOR"-rcord, "Compact Disc" or "CD" and "Laservision"-records, respectively.
In optical record carriers the information is coded in information areas which are generally arranged in tracks and alternate in the track direction with intermediate areas, which information areaas are optically distinct from the intermediate areas and the lands between the information tracks. The information is stored in these tracks in the length and/or the frequency of the information areas and the intermediate areas. The information is recorded by scanning the radiation-sensitive surface of the record carrier by means of a writing beam focused to a radiation spot which is switched in intensity in accordance with the information to be recorded, the intensity variations causing local changes in the surface.
The developments in the field of optically inscribable record carriers increasingly move in the direction of higher signal frequencies. For the write beam this means that the duration of the radiation pulses with which the information areas are formed becomes shorter. However, since the quantity of energy which is required to form an information area remains substantially equal or even increases, the power of the laser pulse must increase. The semiconductor laser will thus have to be driven with shorter lasting and stronger current pulses and preferably also with a larger edge steepness. Upon driving a semiconductor laser with such current pulses switching transients caused, for example, by parasitic capacitances and inductance in the wiring and electro-magnetic radiation effects start to play a role.
The effect of the switching transients can be reduced by continuously passing a d.c.-current or pedestal current, which is slightly smaller than the laser-threshold current, through the semiconductor laser. A laser pulse can then be generated by superposing an information current pulse on the d.c.-current, which pulse has such an intesity that the total current through the laser is larger than the laser-threshold current. Such a laser drive is described in European Patent Specification No. 0,053,974. However, this method has the drawback that due to the d.c.-current a considerable dissipation occurs in the laser in the period between two pulses so that the lifetime of the laser is detrimentally influenced. The problem cited above also occurs in other uses in which a semiconductor laser is to be switched rapidly.